Various water conduit systems exist wherein a buildup of substantial carbonate deposits on the interior surfaces of the components eventually requires cleaning or replacement of the system. Carbonate blockage usually occurs when water hardness in the system exceeds the solubility of the carbonate salts present in the water. This may happen when water is softened by chemical precipitation processes, usually utilizing calcium oxide or calcium hydroxide, followed by pH adjustment of the treated water with carbon dioxide or sodium carbonate in water treatment plants. The softening process results in water saturated with calcium carbonate and can lead to substantial hard carbonate deposits on the interior surfaces of the system. Blockage as thick as three inches in a 12-inch diameter pipe are not uncommon. The calcium carbonate precipitation process may also be utilized to remove iron, manganese, other heavy metals, radionuclides and some dissolved organics from various aqueous streams; all such processes can lead to substantial calcium carbonate deposits on the interior surfaces of the conduits. Substantial blockage leads to substantial flow reduction through the system and requires that the conduits be cleaned or replaced.
Industrial process pipes utilized in the transfer of basic materials such as ammonium hydroxide, sodium hydroxide, calcium hypochlorite and the like, can have substantial blockage due to the buildup of insoluble carbonates. These carbonate deposits are extremely hard, can result in the loss of 50% or more of the designed flow of the solutions through the system, and are very difficult to remove by the usual mechanical means such as hydroblasting, pigging or scraping. This is particularly true if there are valves, elbows or T-joints in the blocked system. Substantial carbonate buildup can be found in any type of pipe or component, including iron, steel, brass, copper, plastic, cement, clay, and the like.
Substantial carbonate blockage can also be found in geothermal and hot springs water conduit systems. Marine vacuum waste piping systems utilizing water for flushing can also accumulate substantial carbonate blockage which requires periodic maintenance cleaning. Substantial carbonate blockage has been found in sewer lines which have groundwater leakage into the carbon dioxide-rich atmosphere of the interior of the sewer. This has resulted in the growth of calcium carbonate stalactites up to 6 inches or more in length and also substantial carbonate blockage around the interior sewer joints.
In the past, mineral acids have generally been employed in an attempt to remove the carbonate blockage. A major safety problem results if a substantial amount of carbonate blockage needs to be removed because the acids will decompose the carbonate deposits, which usually consists of calcium carbonate and/or magnesium carbonate, to rapidly generate sizable volumes of carbon dioxide gas per weight of carbonate. For example, one mole of calcium carbonate (100.09 grams) will generate 22.4 liters of carbon dioxide gas at standard atmospheric conditions. Improper conventional acidic cleaning processes have been known to blow apart pipes and elbows being cleaned of carbonate blockage with explosive force. If too much acid is added to a system containing carbonate blockage, or if the system being cleaned cannot vent the carbon dioxide being formed in a controlled manner, a serious pressure buildup and a potentially explosive condition will most likely result.
U.S. Pat. No. 5,346,626 discloses using organic carboxylic acid polymers and sulfonic acid monomers to remove carbonate scale from heat transfer surfaces at a pH of 4-5. The polymers may be added all at once or in increments. The cleaning process begins at pH 3 and is complete when the solution is pH 5 or higher. The application of this method is directed toward boilers and cooling towers which have only a thin layer of carbonate-containing scale to be removed. U.S. Pat. No. 5,094,757 discloses a circulating acidic coolant solution of pH 4 to clean carbonate scale from vehicle cooling systems. After cleaning and refining, the cleaning coolant is adjusted to pH 9. U.S. Pat. No. 5,021,096 employs a two step process for removing iron oxide and hard water scale from heat exchanger surfaces by first treating with a circulating hydroxyacetic acid/polysaccharide gum solution followed by a circulating citric acid solution. Other art directed toward removal of thin layers of carbonate-containing scale from evaporative or heat exchanged surfaces with various acidic compositions is well known by one versed in that area of art.
The need exists, however, for a safe method to remove substantial amounts of hard carbonate deposits, in a controlled and timely manner, from water conduit systems that demonstrate substantial flow reduction due to carbonate deposits. Because the reaction of acids with carbonate blockage in water pipes to generate carbon dioxide gas is very rapid, the controlled addition of acid and the monitoring of carbon dioxide gas generated is essential to the safe removal of carbonate blockage. Otherwise, carbon dioxide gas pressure buildup or containment can result in potentially serious explosive conditions. Since carbon dioxide gas is heavier than air, it can also pose a hazard to workers removing the carbonate blockage. Carbon dioxide gas must therefore be monitored and controlled in the workplace to ensure a safe work area.